1. Field of the Invention
This invention relates to thermal-dye-bleach agents, and in particular, it relates to non-labile-hydrogen-containing cationic salts of phenylsulfonylacetic acids as bleaching agents for photographic, photothermographic, and thermographic imaging constructions. Constructions employing these thermal-dye-bleach agents are suitable for use as acutance and antihalation systems, bleachable filter dye materials, and in thermal recording processes. In their simplest form, the constructions comprise a quaternary-ammonium salt of a phenylsulfonylacetic acid, capable of generating a carbanion upon thermolysis (i.e., a thermal-carbanion-generating agent) and a dye.
2. Background of the Art
Light-sensitive recording materials suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. Such degradation occurs when a fraction of the imaging light which strikes the photosensitive layer is not absorbed, but instead passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. Light thus reflected may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter in the photosensitive element may also cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. This effect leads to image degradation. Silver-halide based photographic materials (including photothermographic materials) are prone to this form of image degradation since the photosensitive layers contain light-scattering particles (see, T. N. James, "The Theory of the Photographic Process", 4th Edition, Chapter 20, MacMillan 1977).
In order to improve the image sharpness of photographic materials, it is customary to incorporate a dye in one or more layers of the material, the purpose of which is to absorb light that has been scattered within the coating and would otherwise lead to reduced image sharpness. To be effective, the absorption of this layer must be at the same wavelength as the sensitivity of the photosensitive layer.
In the case of imaging materials coated on a transparent base, a light-absorbing layer is frequently coated in a separate backing layer or underlayer on the reverse side of the substrate from the photosensitive layer. Such a coating, known as an "antihalation layer", effectively reduces reflection of any light which has passed through the photosensitive layer. A similar effect may be achieved by interposing a light-absorbing layer between the photosensitive layer and the substrate. This construction, known in the art as an "antihalation underlayer", is applicable to photosensitive coatings on non-transparent as well as on transparent substrates.
A light-absorbing substance may also be incorporated into the photosensitive layer itself in order to absorb scattered light. Substances used for this purpose are known as "acutance dyes." It is also possible to improve image quality by coating a light-absorbing layer above the photosensitive layer of a photographic element. Coatings of this kind, described in U.S. Pat. Nos. 4,312,941, 4,581,323 and 4,581,325, reduce multiple reflections of scattered light between the internal surfaces of a photographic element.
It is usually essential that coatings of antihalation or acutance dyes which absorb in the visible region of the spectrum should completely decolorize under the processing conditions of the photographic material concerned. This may be achieved by a variety of methods, such as by washing out or by chemical reaction in wet processing techniques, or by thermal bleaching during heat processing techniques. In the case of photothermographic materials which are processed by simply heating for a short period, usually between 100.degree. C. and 200.degree. C., antihalation or acutance dyes used must decolorize thermally.
Various thermal-dye-bleach systems are known in the art including single compounds which spontaneously decompose and decolorize at elevated temperatures and combinations of dye and thermal-dye-bleaching agent which together form a thermal-dye-bleach system.
European Patent Publication No. EP 0,377,961 A discloses the use of certain polymethine dyes for infrared antihalation in both wet-processed and dry-processed photographic materials. The dyes bleach completely during wet-processing, but remain unbleached after dry-processing. This is acceptable for some purposes because infrared dyes have a relatively small component of their absorption in the visible region. This absorption can be masked, for example, by using a blue-tinted polyester base. For most applications, however, it is preferable that the dyes bleach completely during dry-processing, leaving no residual stain.
U.S. Pat. No. 5,135,842 describes thermal-dye-bleach constructions employing guanidinium salts of phenylsulfonylacetic acids and polymethine dyes such as IV and V (disclosed later herein). Upon heating, these salts liberate guanidine which nucleophilically adds to the polymethine chain, thereby disrupting conjugation and decolorizing the dye. However, thermal-dye-bleach constructions employing guanidinium salts have relatively short shelf life, are subject to premature bleaching, and, upon heating, display slow bleaching over a broad temperature range.
Many substances are known which absorb visible and/or ultraviolet light, and many are suitable for image improvement purposes in conventional photographic elements sensitized to wavelengths below 650 nm. Triarylmethane and oxonol dyes, in particular, are used extensively in this connection. U.S. Pat. Nos. 3,609,360, 3,619,194, 3,627,527, 3,684,552, 3,852,093, 4,033,948, 4,088,497, 4,196,002, 4,197,131, 4,201,590 and 4,283,487 disclose various thermal-dye-bleach systems which absorb principally in the visible region of the electromagnetic spectrum and as such, are not readily adaptable for use as far-red or near-infrared absorbing constructions. No indication or examples are given of far-red or near-infrared absorbing thermal-dye-bleach systems.
A variety of thermal-base-releasing agents are known and have been used in both diazo- and silver-containing photothermographic materials. However, the purpose of incorporating thermal-base-releasing agents into photothermographic constructions has been to increase the basicity (i.e., alkalinity) of the medium during thermal processing, thereby promoting the development reaction.
For example, U.S. Pat. No. 4,939,064 describes the use of amidine salts of carboxylic acids as base precursors contained within light-sensitive silver halide layers. An amidine base is released by thermolytic decarboxylation of a carboxylic acid to generate a carbanion which removes one or two protons from an amidine salt. The thus released amidine base renders the medium basic so that a polymerization reaction can proceed.
U.S. Pat. No. 4,842,977 describes the use of guanidinium salts as base precursors contained in particles arranged on the outside of microcapsules containing silver halide and a polymerizable compound. The thus released guanidine base renders the medium basic so that a polymerization reaction can occur.
U.S. Pat. No. 4,560,763 describes the use of amine salts of .alpha.,.beta.-acetylenic carboxylic acids as base precursors in photosensitive materials. The amine salts have a labile proton. Again, thermolysis of these materials releases the free base which accelerates reaction of a developing agent for silver halide.
U.S. Pat. No. 4,981,965 describes the use of guanidinium salts of phenylsulfonylacetic acids as base precursors. The diacidic to tetra-acidic base precursors are composed of two to four guanidinium units. In these systems, thermolysis of the salt results in decarboxylation to form a phenylsulfonylmethyl anion. This anion abstracts a proton from the guanidinium salt to release the free base. This base can then provide the alkalinity required for a number of image-forming processes.
U.S. Pat. No. 4,060,420 describes the use of ammonium salts of phenylsulfonylacetic acids as activator-stabilizers in photothermographic systems. In these systems the ammonium species is always a protonated basic nitrogen, and thus has at least one labile hydrogen atom. U.S. Pat. No 4,731,321 discloses ammonium salts of phenylsulfonylacetic acid as base precursors in heat-developable light-sensitive materials.
Japanese Patent Application No.1-150575 discloses thermally-releasable bis-amines in the form of their bis(arylsulfonylacetic acid) salts. Other amine-releasing compounds include 2-carboxycarboxamide derivatives disclosed in U.S. Pat. No. 4,088,496; hydroxylamine carbamates disclosed in U.S. Pat. No. 4,511,650; and aldoxime carbamates disclosed in U.S. Pat. No. 4,499,180.
The above items use an ammonium or guanidinium salt having at least one labile hydrogen atom as the cation for the carboxylic acid anion. In all of the above cases, the ammonium salt serves to release a base; that is, the base is derived from the cationic portion of the molecule. In none of the above items was a quaternary ammonium salt, free of labile hydrogen atoms (such as a tetra-alkyl ammonium salt), used as the cation for a carboxylic acid. In none of the above cases was a non-labile-proton-containing cationic salt of a carboxylic acid used as the basis of a thermographic imaging system or as the basis of an anti-halation coating of a photothermographic imaging system. Finally, in none of the above items was the anionic portion of the salt used as the bleaching species.
U.S. Pat. No. 3,220,846 discloses the use of tetra-alkylammonium salts of readily decarboxylated carboxylic acids to generate a basic medium which promotes coupling of two reactants to form a dye. These materials are taught to be useful in thermography, photography, photothermography, and thermophotography.
U.S. Pat. Nos. 3,684,552, and 3,769,019 disclose the use of tetra-alkylammonium salts of cyanoacetic acid as bleaching agents for light- and heat-sensitive materials. These are unacceptable due to liberation of volatile, potentially toxic materials such as nitriles.
U.S. Pat. No. 4,705,737 describes the use of ammonium phenylsulfonylacetate salts as base generators in heat developable photothermographic layers. Several quaternary-ammonium phenylsulfonylacetate salts are included. The salts are contained in the photosensitive silver halide layer and, after imaging and upon heating, serve to render the photosensitive layer sufficiently alkaline for dye formation, dye coupling, or dye release. The photothermographic layers described are hydrophilic and gelatin-based.